Contact lenses from a wide range of materials are nowadays produced in great volume in highly automated manufacturing facilities. Advantageously, these contact lenses are formed using reusable mould halves, the female and the male, which are normally formed from glass or quartz. When mated (mould assembly), these mould halves define a hollow cavity, which corresponds to the subsequent contact lens shape. Before closing the mould halves, a polymer solution is dosed into the female mould half. After closing the mould halves, UV light is radiated over a mould half, which leads to crosslinking of the lens material in the lens cavity. Subsequently, the lens is removed from the mould half, for example with suction grips or mechanical grippers, and placed in the package.
Because contact lenses are intended for use in the eye, great care is taken to make sure that lenses meet strict quality control standards. To ensure consistent quality of the contact lenses, automatic inspection of the contact lenses using industrial image processing methods may be implemented. The known methods for inspection of ophthalmic lenses are based on bright-field and/or dark-field imaging.
WO-A-2005/054807 discloses a method for the automatic inspection of contact lenses, in particular tinted contact lenses, in an automatic lens manufacturing process, the method comprising the use of a dark-field inspection unit with the preferred dark-field inspection method being the so-called Schlieren method.
EP-A-1248092 further discloses the combination of a dark-field inspection unit and a bright-field inspection unit, preferably the combination of the so-called Schlieren method and the transmitted light method. The contact lens is subsequently observed in a dark-field and bright-field.
WO-A-2004/057297 discloses a method for optically inspecting and detecting defects in an object using two different light sources, in particular using a dark-field setup as the first detection method and a bright-field setup as the second detection method.
WO-A-03/073060 discloses the dual inspection of ophthalmic lenses using at least two different machine vision inspection techniques in the manufacturing process for said ophthalmic lenses, the preferred at least two inspection techniques being bright field and dark field inspection techniques; others being absorptive inspection, structure light inspection, fluorescence inspection and spectral masking.
WO-A-99/32869 discloses a system for inspecting contact lenses which utilizes a light source and an electronic camera for obtaining images of the lens, as well as a series of masks, including a bright-field mask, a dark-field mask and a transition mask which is constituted by fine stripes, such that the light interacts in a constructive and destructive manner at different distances from the mask. The images are taken subsequently using one mask at a time.
EP-A-0686842 discloses a lens inspection system and method using two optimized bright-field illumination zones, i.e. using light at two different grey levels. One grey level for the centre zone and one grey level for the peripheral zone. Whereas the use of phase contrast (another conceivable inspection method) is considered to be hypersensitive, i.e. enhancing cosmetic flaws to an extent that the lens is rejected for being defective.
To improve production yield and more importantly to improve the quality of the ophthalmic lens and the wearers comfort, there is a need to create a more accurate inspection system. In particular, a suitable inspection system should carry out an exhaustive examination of the ophthalmic lenses for accuracy of size, surface defects, tears, peripheral ruptures and inclusions such as bubbles and foreign bodies, as well as small defects at the edges of an ophthalmic lens.